Tetraalkyl titanates are known to be useful as esterification catalysts for the preparation of polyesters by the reaction of diacids or their ester-forming equivalents with diols or their ester-forming equivalents. With little question, the tetraalkyl titanates are preferred catalysts for the preparation of poly(butylene terephthalate) and copolyetherester elastomers having butylene terephthalate hard segments and polyether soft segments.
While the tetraalkyl titanates are highly effective per se, numerous investigators have studied modification of these catalysts by reacting tetraalkyl titanates with compounds of other metals to form complex compounds or by using titanate catalyst in combination with catalytically active compounds of other metals. These studies had objects such as increasing the activity of the titanates, reducing degradative side reactions, improving color and product quality, etc.
U.S. Pat. No. 3,326,965 to Schultheis et al. discloses the preparation of polyesters from diacids and diols in the presence of a wide variety of titanium compounds or zirconium compounds or mixtures thereof. Although tetraalkyl titanates and tetraalkyl zirconates are disclosed as members of extensive lists of titanium and zirconium compounds, no mixtures are exemplified much less ratios of titanium to zirconium and no advantage is taught in the patent for such mixtures. The disclosures of U.S. Pat. No. 3,326,965 are in contrast to the disclosures of U.S. Pat. No. 2,822,348 to Haslam which teaches that tetraalkyl titanates are highly effective esterification catalysts and tetraalkyl zirconates are inactive. This patent does not suggest the use of combinations of tetraalkyl titanates and zirconates.